I've never really tried to stretch the battery life, it does pretty good as is.

Gell Batteries are relatively easy to deal with, and if you look, you'll find 4V units so you can stack up 16V easily. However, they certainly don't have as much watts per bulk or pound, as NIMH.  If I were you, I would look at some NIMH D cells.  I'm getting 3.8AH now in cells just a bit larger than AA cells.

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